Wind turbines start operating at wind speeds of 4 to 5 metres per second (around 16 kilometres an hour - ie. a "gentle breeze", where leaves and small twigs on trees are in constant motion) and reach maximum power output at around 15 meters/second (around 54 kilometres per hour - ie. "stong breeze" to "near gale" - when whole trees are in motion, and it becomes hard to walk against the wind). At very high wind speeds, i.e. gale force winds, (25 metres/second, 90+ kilometres/hour) wind turbines shut down.